A family of Microsoft word processing software products for creating web, email, and print documents.
If you're using a desktop version of Word for Windows or Word for Mac, the SmartArt button should be on the Insert ribbon. In Word for Windows, it's in the middle of the Illustrations group:
If you don't see it there, try doing an Online Repair of Office by the steps at the Microsoft page https://support.office.com/en-us/article/Repair-an-Office-application-7821d4b6-7c1d-4205-aa0e-a6b40c5bb88b .
SmartArt isn't offered in Word Online (in a browser) or Word for Android, but it is in PowerPoint for those Office versions. To use SmartArt in PowerPoint for Android, though, you must subscribe to Microsoft 365 Personal or higher.